In recent years, device control systems for controlling devices by selectively operating, through a remote controller, device control graphics (icons) composed of screen display data and characters indicating functions of the devices on a TV (Television) screen have made their appearance. Also, network systems with digital devices such as a DVD connected thereto using IEEE 1394-1995 for transmitting and receiving video/audio data have been emerging.
One example of conventional network control systems is disclosed in Japanese Patent Laying-Open No.H9-149325 (1997-149325).
This conventional network control system is now described below.
Similarly to digital interfaces according to IEEE 1394 standards, etc., AV units are connected to each other without switching by a bilateral packet communications system with a serial bus that periodically provides an equal communication opportunity to the devices.
Here, each AV unit stores its own screen display data. By request from a controller having a graphic display function (television receiver), each AV unit transmits the screen display data to the controller. The controller displays the received screen display data. The controller further has the functions of inquiring for data required for indicating the AV unit connected thereto and controlling the display screen based on the screen display data from the AV unit. Each AV unit has a recording medium for storing the screen display data, and a function of selecting the appropriate screen display data in response to the inquiry from the controller for the screen display data.
In the above structured network control system, the screen display data is held in each device (AV unit), and outputted according to a display request from the controller (television receiver). Graphics unique to each device (AV unit) are thus displayed on the screen of the controller.
In the above described structure, however, when the display on an operating screen is to be changed due to internal state changes in the device, the device cannot report its change of state to the controller, thereby causing a mismatch in display screen information between the controller and the device. For this reason, the controller cannot correctly provide a user with device operation information.
In view of the above problem, an object of the present invention is to provide a network control system in which a device can quickly report to a controller about a change of the device's internal state that causes a change of display on an operating screen to ensure the controller and the device can share the same state information. Such network control system is herein achieved with simple structure and small transmission load on a communication path.